Δεν ξέρω αν μας συμφέρει να μείνουμε μέχρι αργά στο γραφείο σήμερα.

Here αν introduces an indirect yes/no question, so it means whether or if:

• Δεν ξέρω αν... = I don’t know whether... / I don’t know if...

This is not a conditional if like If it rains, we’ll stay home. It is more like asking internally: Is it worth it or not?

A useful contrast:

• Ξέρω ότι είναι αργά = I know that it is late
• Ξέρω αν είναι αργά = I know whether it is late

So in your sentence, αν is the correct word because the speaker is uncertain and is talking about a yes/no issue.

Συμφέρει comes from συμφέρω and here it means something like:

• it is advantageous
• it is worth it
• it is in our interest
• it makes practical sense

So μας συμφέρει is not just about liking the idea. It suggests a practical calculation: time, effort, benefit, convenience, cost, or strategy.

In this sentence, the meaning is roughly:

• I don’t know whether it’s worth it for us to stay late at the office today

Greek often uses verbs like συμφέρει impersonally, without a dummy subject.

English needs a subject, so it says:

• It is worth it
• It suits us
• It is useful

Greek often just says:

• συμφέρει
• βοηθάει
• φαίνεται

So in μας συμφέρει να μείνουμε..., Greek does not need an extra word for it. The idea of what is advantageous is supplied by the να clause: να μείνουμε μέχρι αργά...

Μας means to us / for us here.

With συμφέρει, Greek uses a weak pronoun to show who benefits or for whom something is advantageous:

• μου συμφέρει = it suits me
• σου συμφέρει = it suits you
• μας συμφέρει = it suits us

As for position, weak object pronouns in Greek normally come before the finite verb:

• μας συμφέρει
• του αρέσει
• σας φαίνεται

So μας συμφέρει is the normal order.

Because Greek commonly uses να + verb to express an action that is being considered, wanted, allowed, possible, useful, necessary, and so on.

Here, να μείνουμε tells you what action may or may not be advantageous:

• μας συμφέρει να μείνουμε = it is advantageous for us to stay

This is a very common pattern in Greek:

• Θέλω να φύγω = I want to leave
• Μπορώ να έρθω = I can come
• Πρέπει να δουλέψουμε = We must work
• Συμφέρει να περιμένουμε = It’s worth waiting

So να introduces the dependent verb phrase.

This is about mood and aspect.

1. Not μένουμε

   • μένουμε is indicative: we stay / we are staying
   • After να, you need the dependent form, not the plain indicative

2. Why μείνουμε

   • μείνουμε is the aorist subjunctive of μένω
   • It views the action as a whole event: the decision to stay late today

3. Why not να μένουμε

   • να μένουμε would sound more ongoing, habitual, or repeated
   • It could suggest something like to be staying late regularly or to keep staying late

In this sentence, the speaker is talking about one specific situation today, so να μείνουμε is the natural choice.

The subject is built into the verb ending.

• μείνουμε = we stay / we remain in this να-clause form

Greek often leaves subject pronouns out because the verb ending already shows the person and number.

So:

• να μείνουμε already means for us to stay / that we stay

You would add εμείς only for emphasis or contrast, for example:

• να μείνουμε εμείς, όχι αυτοί = for us to stay, not them

Without emphasis, leaving it out is more natural.

Μέχρι αργά means until late or, more naturally in English here, late in the sense of stay late.

• μέχρι = until / up to
• αργά = late

So:

• να μείνουμε μέχρι αργά = to stay until late
• Natural English: to stay late

Greek often uses this kind of phrase where English prefers a simpler adverbial expression.

Στο is a contraction of σε + το.

• σε = in / at / to
• το γραφείο = the office

So:

• σε το γραφείο becomes στο γραφείο

This contraction is standard and very common in everyday Greek.

Also, after σε, the noun is in the accusative. With a neuter singular noun like γραφείο, the form looks the same as the nominative, so you do not see a visible change.

Depending on context, στο γραφείο can mean:

• at the office
• in the office
• sometimes to the office

Here it means at/in the office.

Yes, Greek word order is quite flexible.

In this sentence, σήμερα at the end sounds natural and simply adds the time frame at the end of the thought:

• Δεν ξέρω αν μας συμφέρει να μείνουμε μέχρι αργά στο γραφείο σήμερα.

But Greek could also move σήμερα earlier for emphasis or rhythm, for example:

• Δεν ξέρω αν μας συμφέρει σήμερα να μείνουμε μέχρι αργά στο γραφείο.
• Σήμερα δεν ξέρω αν μας συμφέρει να μείνουμε μέχρι αργά στο γραφείο.

These versions are all possible, but the focus shifts slightly:

• final σήμερα = neutral, natural
• earlier σήμερα = stronger emphasis on today

So the sentence you have is a very normal word order.